1. Field of the Invention
The present invention relates to a method for producing mother rods useful for the production of optical fibers wherein the rods are manufactured without going through the step of fusing or melting the glass.
2. Brief Description of the Prior Art
Optical fibers are used for various applications including, for example, extremely short distance light transmission elements such as gastrocamera fiber-scopes, long distance light transmission systems such as optical communication systems, and the like. They are mainly made of a high content silica glass containing a dopant for adjusting the refractive index of the glass, if required.
Conventionally known methods for producing such optical fibers are (i) a method of melting the starting material in a crucible or port to form fibers from the molten glass, (ii) a method of forming a high silica glass membrane on the inside wall of a quartz tube by means of the CVD Method (Chemical Vapor Deposition Method), collapsing the thus formed tubular membrane and then drawing the same to form fibers, and (iii) a method of forming glass soot by means of the CVD Method, axially depositing and sintering the soot and then drawing the resultant sintered body to form fibers.
However, these known methods have the following disadvantages. By the method set forth in (i) above, optical fibers of high purity cannot be produced for the reasons that a glass of high silica content is difficult to prepare; that difficulties are encountered in purification of the starting material; and that contamination with the ingredients of the used pot cannot be obviated. The method set forth in (ii) above is disadvantageous because it does not lend itself to mass producibility; a large-sized mother rod and a multicomponent glass, containing a relatively large amount of Na cannot be produced, the yield is reduced when P, Ge, B or the like is used as the dopant element since a thermal oxidation reaction is utilized in this method, and because an expensive production facility is required. The method set forth in (iii) is disadvantageous in that a multicomponent glass cannot be produced; it does not lend itself to mass producibility; and an expensive production facility is required. A further disadvantage of the known methods is that a glass body of an arbitrary shape cannot be formed by any of the methods (i), (ii) and (iii). Furthermore, the products made by these known methods are inevitably increased in cost, since in either of the methods (ii) and (iii) silicon oxide is formed under a high temperature condition, and in the method (i) the glass must be adequately melted at a high temperature.
On the other hand, a method of preparing a porous glass at a relatively low temperature is known. In this method, a high content silica gel containing silica or any other dopant oxides is obtained by hydrolyzing silicon alkoxide, with the optional addition of alkoxides of any desired elements.
Also well known in the art is a method wherein a liquid mixture mainly composed of silicon alkoxide is dropped onto the liquid surface of an appropriate liquid or drawn in the liquid followed by sintering with heat to produce a film-shaped or fibrous glass product. However, this method has serious disadvantages in that the shape of the products produced thereby is limited to an extremely narrow range and the product is unstable in shape and lacks uniformity.
The state of the art is exemplified by Japanese Pat. Publication No. 6604/1973, Japanese Pat. Laid-Open No. 34219/1976 and M. Yamane et al, Journal of Materials Science, 13 (1978), pages 865-870.